Exciter lamp supply



H. BELAR EXCITER LAMP SUPPLY April 27, 1948..

Filed Oct. 31,1942

' FIG-Z.

' LINE vow/16E: Y

F115 JMENT VOL, 77765 rzn snnruns v Z'mnentor: Herbert Bel/an" (Ittorneg Patented Apr. 27, 1948 Herbert Belar, Palmyra, N. 3., assignor to Radio Corporation of America, a corporation of Dela-.

ware

Application October 31, 1942, Serial No. 464,131.

3 Claims.

This invention relates to the current supply for an eXciter lamp such as used with photoelectric apparatus and particularly to photoelectric apparatus of the sound reproducing type.

It is customary, in sound reproducers and analogous apparatus, to use an exciter lamp of the low voltage, high intensity type, for example, a volt, 7 ampere lamp is quite usual in sound reproducers and lamps of other voltages and currents of this general order are used. The aver-.

age motion picture theatre is supplied with 110 volt, 60 cycle alternating current and if this current is stepped down by a transformer to operate the exciter lamp, a very noticeable 120 cycle hum will be produced in the sound reproducer, although the low voltage, heavy filament exciter lamps are not as bad in this respect as 110 volt lamps with a thinner filament. In order to avoid this hum, many expedients have heretofore been provided. Storage batteries have been used but the space occupied by such batteries and the expense oi upkeep are quite considerable. High frequency current from an oscillator has been used on small exciter lamps but the cost of an oscillator and its proper power supply render this arrangement impractical for the large exciter lamps. Current for the exciter lamps has also been'rectified and filtered and although this is an accepted commercial practice, the cost of the rectifier and filter is quite considerable. In inexpensive equipment it has been customary to use alternating current on the exciter lamp and to provide the amplifier or the sound reproducer or both with such characteristics that frequencies below about 150 cycles would not be reproduced at a high level. This, of course, eliminates the A.-C. hum but at the cost of the lower frequencies from the sound record. a

It will be apparent that allthe foregoing arrangements have their advantages and that each of them has certain disadvantages. In my improved construction, the high cost of either the oscillator, the storage battery or the rectifier is avoided. The circuit is much simplified and at the same time a result is produced which is comparable with that produced by a good rectifier and filter or a storage battery of reasonable size.

In my improved arrangement, I supply the exciter lamp with current through a device which will give a more or less fiat topped characteristic to the current so that the filament current remains substantially constant during a considerable portion of each half cycle. If the flat topped portion of the current wave is a. sufilciently large percentage ofrthe half cycle, the thermal inertia of the filament will tend to smooth out theinterruption between that half cycle and the corresponding portion of the next half cycle and a substantially uniform illumination with very little hum producing characteristic will result.

One object of the invention is to provide an exciter lampsupply which'will prevent hum in the. sound reproducer. 7

Another object of the invention is to provide a relatively simple exciter lamp supply which will provide uniformcurrentfor the exciter lamp.

Another object of the invention is to provide means of controlling the current through the exciter lamp.

Other and incidental objects of the invention will be apparent to those skilled in the art from a reading of the following specification and an inspection of the accompanying drawing in which:

Figure 1 is a diagrammatic illustration of my improved exciter lamp supply; a

Figure 2 shows approximately the wave form of alternating current at a low voltage;

Figure 2 shows the wave form of alternating current at a high voltage;

Figure 3 shows the wave form of the current of Fig. 2 after it has been modified by my apparatus; i t

Figure 3A shows the wave form of the current of Fig. 2A after it has been modified by my apparatus;

Figure 4 shows the filament temperature produced by current of the form shown in Fig. 3; and

Figure 4A shows the filament temperature curve produced by current of the form shown in Fig. 3A. Referring first to Fig. 1, the alternating current is supplied to the exciter lamp 10 having a filament ll through a control lamp It. The control lamp i2 is provided with relatively heavy leads It and with a plurality of relatively fine filaments I l. The filaments l4 preferably operate at a rather high temperature in vacuum so that the thermal emission from the filaments will follow the radiation laws in their temperature variation. With the usual types of resistance lamps, the filaments operate at a relatively low temperature in a thermally highly conductive atmosphere, such as hydrogen. This type of resistance lamp does not tend to change its losses as rapidly with change in current as the vacuum type above reierred to. Any type of resistance or current control device may be used which effectively limits the current to a constant value and the greater the voltage drop in the current control device,

the more nearly the current will become constant. As indicated in Fig. 3, if the voltage drop across the resistance device I4 is relatively low, the current Waves will have flat tops but very rounded corners, while if the voltage drop is greater than as indicated in Fig. 3A, the wave shape will become substantially square with very .slightly rounded corners.

Although the current reverses in direction each half cycle, this reversal does not itself afiect the filament temperature. As shown in Fig. 4, when a current of the .shape indicated in Fig.3 'is used, the filament temperature willremain substantially constant with a slight drbp between the. half cycles due to the rounded corners of the wave shape.

If a larger voltage drop occursinthecontrdl device l4, then the filament temperaturewill follow the curves shown in-Fig. 4A where the drop in current is quite abrupt butthefilament temperature as indicated by1the dotted line changes only very slightly .during. the period of the rounded corners of the wave shape.

In order to. secure maximum reduction of hum it is desirable, for the reasonsijust given, to secure a maximum of voltage drop in the device M in proportion to the voltage. drop'in the exciter lamp filament II. 'For example, theapparatus may be connected across the 110 volt supply line and if, for examplea volt .exciter lamp is used, then the. resistance device'would'bed'esigned to pass..the current requiredflby theilamp and to provide avoltage .drop of 100 volts. This arrangement provides a very square ,wave shape and practically completelyeliminates any hum due to the exciter lampas this hum would'be brought down to ,al'evel below the normal hum level. of the-amplifier.

In choosing the proper voltage to be applied and the voltage .drop to beabsorbed in the device I 4, a number of factorsshould'be considered, such as power loss .ascompared with the reduction of hum and the ,amountjof hum reduction necessaryinorder to'produce satisfactory results with the particular amplifier .and sound reproducer used. It would beunnecessary toreduceitheihum materiallybelow the humproduced by the amplifier and hum reduction below this level would be uneconomical asit would increase the cost of the current control device and increase thepower loss.

It will be apparent to those skilled in theart that thecurrent control device does not need to be of the type shown and described as vacuum tube circuits are available for controlling current which willaccomplishsubstantially the same result. The form of theinvention shownyhowever, is at present the preferred form due'to .the commercial lack of vacuum tubes of suflicient size and number to satisfactorily control commercial exciter lamps and due to the relatively greater cost of such tubes if available.

I claim as my invention:

1. A photoelectric sound reproducing system, including the combination of an exciter lamp, terminal connections to a source of alternating vnltageiforsaid lamp, andaresistorpf low thermal inertia connected inseries'between said lamp and said terminal connections and having a resistance which varies with the amplitude of said voltage during a period which is small in relation to a half cycle of said voltage.

2.:A' system according to claim 1 wherein said resistor is contained in an evacuated envelope REFERENCES CITED The "following references are of record in the file of this patent;

UNIIED swarms PATENTS Number Name Date 673,935 -Wurts May 14, '1901 r 685,528 "Petter 1 Oct. 29, 1901 709,023 Liebet al Sept. 16, 1902 7 902,069 Gernsbaok Oct. 27, 1908 1,023,485 Thowless Apr. 16, 1912 1,416,007 Eschhclz May 16, 1922 1,457,851 Nesbit 1 June 5,1923 1,924,058- Edwards Aug. 22, 1933 1,945,726 Brockway Feb.'6, 1934 1,947,484 Miller Feb. 20, 1934 1,973,360 SiX'et al Sept'll, 1934 2,036,071 *Mucher Mar. 31, 1936 2,042,540 Manders June 2,1936 2,092,096 Swedl-und V Sept. 7,.1937 2;120,527 Parisi June 14, 1938 2,122,436 Pirani et a1; July 5, 1938 2245,8137 Stoddard et al. June 17,1941 2,360,233 I-Iussey n Oct. 10, 1944 2,386g903 Lutomirski Oct. 16, 1945 FOREIGN PATENTS Number Country Date 107,435 Australia May 25, 1939 290,244 Great Britain Nov. 29, 1928 434,374 France Feb. 1, 1912 836,418.

France Jan. 18, 1939 

